Gastrola

Gastrola, the name of the stage in development which is passed through by all Metazoa or multicellular animals. It consists of a simple spherical or egg-shaped sac, composed of a double layer of cells with an opening (the blastopore) at one end. In one group of animals (the Gastreadse) the adult condition is not more advanced than this. [See Blastosphere for the general sketch of the The most important characters only are mentioned. early development.]